Stepping motor

ABSTRACT

A multiphase rotary stepping motor of the type having a shaft with the rotors distributed axially thereon, and a housing carrying the axially distributed stators, the rotors and stators having their respective pole-forming elements separated by axial air gaps. The shaft is mounted for rotation in the housing by a bearing arrangement which includes an axially fixed bearing, such as a duplex bearing, located at one end of the motor to restrict axial motion of the shaft, and an axially floating bearing located at the other end of the motor and freely allowing the shaft to move axially as a result of thermal expansion and contraction. Mounted on the free end of the shaft are inertial damping means in the form of an annular mass concentric with the shaft, with a spring and disks of low friction material, such as Teflon, linking the shaft and annular mass in a predetermined pressure and friction relationship.

United States Patent 1 Lyman, Jr.

Sept. 4, 1973 1 STEPPING MOTOR [75] Inventor: Frank Lyman, .lr.,Cambridge, Mass.

[73] Assignee: Cambridge Thermionic Corporation, Cambridge, Mass.

[22] Filed: May 18, 1971 [21] Appl. No.: 144,494

[52] [1.8. CI 310/49, 310/90, 310/112 [51] Int. Cl. H02k 37/00 [58]Field of Search 310/49, 90, 268,

[56] References Cited UNITED STATES PATENTS 3,469,123 9/1969 lnaba310/49 3,429,224 2/1969 Osburn 308/178 3,456,138 7/1969 Huber 310/492,982,872 5/1961 Fredrickson 310/163 2,206,323 7/1940 Hughes 308/1782,446,290 8/1948 Lovegrove 310/49 FOREIGN PATENTS OR APPLICATIONS1,032,040 6/1958 Germany 308/178 Primary ExaminerR. SkudyAttorney-Sewall P. Bronstein, Robert F. OConnell et al.

[57] ABSTRACT A multiphase rotary stepping motor of the type having ashaft with the rotors distributed axially thereon, and a housingcarrying the axially distributed stators, the rotors and stators havingtheir respective pole-forming elements separated by axial air gaps. Theshaft is mounted for rotation in the housing by a bearing arrangementwhich includes an axially fixed bearing, such as a duplex bearing,located at one end of the motor to restrict axial motion of the shaft,and an axially floating bearing located at the other end of the motorand freely allowing the shaft to move axially as a result of thermalexpansion and contraction. Mounted on the free end of the shaft areinertial damping means in the form of an annular mass concentric withthe shaft, with a spring and disks of low friction material, such asTeflon, linking the shaft and annular mass in a predetermined pressureand friction relationship.

5 Claims, 9 Drawing Figures STEPPING MOTOR BACKGROUND OF THE INVENTIONThe present invention relates generally to rotary stepping motors, andmore particularly to mounting and inertial problems associated with therotating portions thereof.

Stepping motors or impulse motors, which rotate through a predeterminedangle upon application of a pulse of current, have been useful in fieldswhere positive, accurate, discrete movements reliably responsive torelatively high frequency pulses are desired. Applications involving thefrequent repositioning of a mechanical member to successively differentpositions are ideally adapted for the use of a stepping motor. Anexample that is of increasing importance is the numerical or pulseprogrammed automated control of machinery. The pulse instructions areconverted directly to accurate mechanical motions by the stepping motor.

Typical stepping motor construction provides, for each phase, a set ofrotor poles, a set of stator poles alignable with the rotor poles and acoil for generating a magnetic field. The magnetic field produces forceswhich bring the rotor poles into alignment with the stator poles, whichis the lowest reluctance configuration. Stepping motors are normally ofmultiphase construction, which means they employ a plurality ofalignable sets of rotor and stator poles, the rotor or stator poles ofsuccessive phases being offset by an angle that is generally apredetermined fraction of the angle between adjacent poles in a singlecombination. As an example, a five phase motor with twenty rotor andstator poles in each phase spaced 18 apart would have its five phasesoffset successively by one fifth of 18 or 3.6. The phases are pulsedsequentially to bring each rotorstator combination from non-alignmentinto an aligned configuration, thereby to rotate the shaft to which torotors are commonly joined. Each pulse in the example corresponds to 3.6rotation, which is the precision of control available.

The rotating parts of a stepping motor, i.e., the shaft and rotors, aresubject to mounting and dynamic problems not encountered in othermotors. The mounting problem arises because, in common stepping motorconstruction, the rotor pole pieces extend radially from a shaft andinto an axial space between opposed pole faces on the stator polepieces. An axial air gap, through which magnetic flux passes, existsbetween the adjacent rotor and stator pole faces. Reduction of the widthof these air gaps increases the aligningforces of the magnetic field andthus increases the motors torque and power. Reduction in width of theaxial air gap, however, necessarily restricts the axial freedom of themotor shaft if collisions between rotor and stator are to be avoided.Various proposals have dealt with the problem of reduced axial shaftmounting tolerance brought about by reduced axial air gaps. Oneproposal, disclosed in Inaba U.S. Pat. No. 3,469,123, isolates the axialmotion of the rotors with-thrust bearings, while permitting the shaft onwhich the rotors ride to move axially. This proposal, however, resultsin a fairly complicated bearing arrangement which still must allow forthe effects of thermal expansion and contraction of the rotors soconstrained. When enough axial play is retained to allow for theseeffects, new mounting problems arise if gap widths are not to beadversely affected.

The dynamic problem of stepping motors results from their angularcontrol requirements, and from the fact that they are advanced by theapplication of a pulse of current. Higher speeds and larger torques callfor very fast pulse rise times, but with limitation of peak current, asexemplified by the copending application of William McDonald, Ser. No.139,721, filed May 3, 1971. The application of discrete pulses ofcurrent to the stepping motor results in uneven forces applied to therotors, and a certain loss of smoothness of operation. The conventionalmanner of dealing with this problem is to increase rotational inertia tobenefit from the flywheel effect. In stepping motors, however, thisapproach cannot be used with much success since one major advantage of astepping motor is the precise rotational control that can be obtained,allowing the shaft to be stopped with a positional accuracy of theoffset between successive phases, e.g., 3.6 for the five phase, twentypole motor previously given as an example. Moreover, low rotationalinertia is desirable in a stepping motor to enable it to be startedquickly, at a high pulse rate.

SUMMARY OF THE INVENTION Objects of the present invention are to providea stepping motor which eliminates the mounting and inertial problems ofthe prior art, and which is simply, easily and inexpensivelyconstructed.

The stepping motor according to the invention is of the multiphase typehaving a plurality of phases distributed axially along a shaft. Inconventional fashion, each phase comprises a rotor rotationally securedto the shaft and having radially extending pole pieces presenting axialpole faces, and a stator around the rotor and having radially extendingpole pieces presenting axial pole faces confronting the rotor pole facesand a coil to develop a flux path through rotor and stator upon passageof current in the coil. Housing means support the stators, and accordingto the invention, bearing means mount the shaft within the housingmeans, the

bearing means comprising an axially fixed bearing,

such as a duplex bearing, located at one end of the motor andrestraining axial motion of the shaft. At the other end of the motor,the bearing means comprises an axially floating bearing allowing theshaft to move axially. Without axial play in the shaft,.its accuratepositioning during mounting is simplified, and tolerances between rotorand stator pole pieces are-maintained more accurately. I

Mounted on the floating end of the shaft are damper means frictionallyconnecting an annular mass to the shaft for inertial control and energyabsorption. In one practical version of the invention, the damping meanscomprises a flange, extending radially from the shaft, disks of frictionmaterial such as Teflon positioned on each side of said radial flange,the annular mass having a pair of faces for engagement with saidfriction disks and spring means urging said faces together.

These and other objects and novel aspects of the invention will beapparent from the following description of a preferred embodiment.

DESCRIPTION OF THE DRAWING FIG. 1 is an axial section of a steppingmotor according to the invention; 4 FIG. 1A is a portion of FIG. 1 togreater scale;

FIG. 2 is a section on line 22 of FIG. 1 showing details of rotorconstruction;

FIG. 3 is an end view of a stator member;

FIG. 4 is a section on line 4-4 of FIG. 3;

FIG. 5 is an end view of a rotor member;

FIG. 6 is an end view, as from the righthand end of FIG. 1, of dampingmeans according to the invention;

FIG. 7 is a section on line 7-7 of FIG. 6; and

FIG. 8 is a side view of a portion of the damping means of FIGS. 6 and7.

DESCRIPTION OF THE PREFERRED EMBODIMENT The illustrated stepping motor10 is of multiphase construction with five separate phases each havingtwenty sets of alignable rotor and stator poles and therefore providingangular control to one-hundredth of a revolution, or 3.6. The steppingmotor 10 comprises a housing assembly 12 with end blocks 14 and I16serving to rotatably mount a shaft 18 in a manner to be described below.Between end blocks M and 16, and distributed axially along shaft 18, arethe five phases A through E (only phases A, B, and E being shown in FIG.1 The phases A through E comprise rotors 20A through 20E carried byshaft 1% and stators 50A through 50E secured to end blocks 14 and 16 bymeans of four bolts 80. The stator poles of successive phases are offsetby an angle of 3.6. With the-exception of this angular offset, thephases A through E are similarly constructed and the followingdescription of phase A serves to describe the other phases as well.

Each rotor, such as 20A, has its individual poles formed by rotor polepieces 21A which extend radially and present axially looking pole faces22A and 23A (FIG. 1A). The individual stator poles are formed by opposedstator pole pieces 51A and 52A presenting pole faces 53A and 54Aadjacent to the rotor pole faces 22A and 23A with axial gaps gtherebetween. Stepping motor rotation takes place when current throughstator coil 62A creates a magnetic potential between stator pole pieces51A and 52A, attracting rotor pole piece 21A into alignment therewith.

Rotor 20A of phase A comprises a non-magnetic cylindrical spacer sleeve24A mounted by key 86 on shaft 18 in abutment with an enlarged hub 82formed thereon. Sleeve 24A is provided with a shoulder 26A which mountsa rotor disk 28A (FIG. 5)' having the rotor pole piecesZlA formedthereon. Rotor disk 28A, which is made of a magnetic material such assilicon iron, is secured, for example with silver solder, to spacersleeve 24A with rotor pole pieces 21A in correct angular orientationwith respect to key 86. The rotors of phases B through D are identicallyconstructed, but the rotor of phase B has a spacer sleeve 24E which isshorter than spacer sleeves 24A through 24D to make room for a retainernut 88 which is threaded on shaft 18 and which secures all the spacersleeves against axial movement on shaft 118.

Stator portion 50A of phase A comprises two opposed stator disks 56A and58A of magnetic material such as silicon iron and having stator polepieces 51A and 52A, respectively, formed thereon like teeth on a facegear (FIGS. 3 and 4).

Stator disks 56A and 58A are provided with facing annular cavities 64Aand 66A and are separated by a spacer plate 60A of magnetic materialsuch as silicon iron and which has a central aperture 68A coincidingwith the outside edge of cavities 64A and 66A. Coil 62A is securedwithin the annular space so provided. One of the stator disks, such as56A (FIGS. 1, 3 and 4) is provided with a slot 70A to enable leads 72Aof coil 62A to pass into a wiring conduit secured to end blocks 14 and16 and provided at one end with a connector socket 92 for connection tothe circuits which drive stepping motor 10. A silicon rubber plug 74A(FIG. 1) closes slot 70A. As shown in FIGS. 2 and 3, the stator disks56A and 58A and the spacer 60A are substantially square, being providedwith corner holes 80h to receive the bolts 80 securing them to thehousing end blocks 14 and 16. In addition, as shown in FIG. 3, statordisks 56A and 58A are provided with four diagonally located observationholes 94 situated between stator holes 51A or 52A for the purpose ofmeasuring the portion of the rotor and its squareness, and fourdiagonally located observation holes 5 6 situated outside coil cavity64A or 66A for the purpose of measuring the portions of the statorplates mating surfaces at the corners thereof. The observation holes 4and 96 are plugged, after measurement, with flat head set screws (notshown).

The stator disks 56A, 58A of phase A abut end block 14 on one side andthe stator disks 56B, 58B of phase B on the other side, and'the statordisks of the remaining phases are similarly in abutment. To avoidmagnetic interaction of adjacent phases as a result of this abuttingrelationship, the stator disks 56A, 58A are provided on their abuttingsurfaces with shallow outer recesses 76A, 78A which form air gapsbetween adjacent phases with sufficient reluctance to prevent the fluxof one phase from linking the other phases to any substantial extent.

As can be seen from the foregoing description, the phases A through Eare constructed in layers. Accordingly, assembly of phases A through Etakes place by assembling end block 14 and shaft 18 as described belowand then by stacking successive layers for rotor and stator of the fivephases and then by securing the stacked layers together.

As indicated in FIG. I and as described above, axial air gaps g areformed between the rotor pole faces 22A and 23A and the stator polefaces 53A and 54A of phase A, and of the other phases as well. It isdesirable to make gapsg as narrow as possible to increase the attractingforces on rotor pole pieces 21A for a'given passage of current in thecoil 62A. As the gaps g are made narrower, the axial positioning ofrotor pole pieces 21A becomes more critical. End thrust on shaft 18,wear in the mount of shaft 18, and thermal expansion of shaft 18 allcontribute to axial movement of rotor pole pieces 21A and prevent usageof narrow gaps g.

-To solve these problems of axial positioning of the rotor pole pieces,the present invention mounts shaft 18 with an axially fixed bearing 100located at one end of the five phases A through E, and an axiallyfloating bearing 102 located at the other end of the phases A through E.The axially fixed bearing 100 prevents axial displacement due to endthrust on shaft 18; the axially floating bearing 102 permits the freeend of the shaft to move axially to allow for thermal expansion andcontraction, thus eliminating any need for providing axial play in thesupport of shaft 18.

As illustrated in FIG. 1, the axially fixed bearing 100 is a duplexbearing comprising a pair of single row bearings 104 and 106 mountedface to face under preload conditions with inner races secured againsthub 82 and around shaft 18 by retainer nut 108 threaded thereon and withouter preloading races secured in a cylindrical recess 109 in end block14 by retainer nut 110 threaded therein. A suitable practical version ofbearing 100 is provided by the duplex bearing manufactured by Fafnir asModel 7203-DU.

The axially floating bearing 102 is a ball bearing, such as Fafnir ModelZOIKDD, whose inner race is secured to shaft 18 and whose outer race 112slides within a cylindrical hole 114 provided in end block 16. Whateverthermal expansion and contraction may develop is accommodated by axialmotion of bearing 102. This arrangement not only eliminates frequentsources of axial displacement of shaft 18, but also enables shaft 18 tobe very accurately placed during assembly since there is no need toprovide axial play.

As shown in FIG. 1, stepping motor further comprises a damping means 120mounted on shaft 18 within a cylindrical recess 122 provided in endblock 16. Referring to FIGS. 6, 7 and 8, damping means 120 comprises aninner sleeve 124 having set screws 126 to secure it to shaft 18.Rotatable about sleeve 124 is an annular mass member 127 concentric withshaft 18 and comprising members 128 and 130 secured together by bolts132. A frictional linkage is provided between sleeve 124 on the one handand annular mass member 127 on the other hand by means of the frictionallinkage 134 shown in FIG. 7. As illustrated, sleeve 124 has an integral,radially extending flange 136 which is disposed between an inwardlyextending radial flange 138 provided on annular member 130, and a washer140 resiliently urged towards flange 138 by a compression spring 142pushing against a flange 143 on member 128. Separating sleeve flange 136from flange 138 and washer 140 are two disks 144 of friction reducingmaterial, such as Teflon.

The inertial mass of annular members 128 and 130, washer 140 and spring142, is thus frictionally connected to shaft 18. When shaft 18 isrotated with sufficient acceleration to cause slippage at the frictionalinterfaces, then frictional energy will be absorbed and the inertialforces on shaft 18 will be reduced, the energy of the rotating massbeing transferred to heat by friction. When acceleration of shaft 18 istoo low to cause slippage, damper 120 behaves like a simple rotatinginertial mass in the manner of a flywheel.

From the foregoing description, it can be appreciated that damper 120permits stepping motor 10 to be accelerated very quickly, both onstarting and stopping, since frictional slippage lowers the effectivemoment of inertia of the rotating parts. When the motor 10 is running ata fairly uniform speed, however, damper 120 increases the moment ofinertia of the rotating parts and, acting like a flywheel, smooths outthe impulses of power applied to the shaft 18 for steadier operation.Any energy impulses causing the speed of shaft 18 to deviate markedlyfrom its average speed will result in frictional slippage and will causea portion of such energy to be absorbed by damper 120 rather than to betransmitted through shaft 18. The precise relationship of mass,coefficient of friction, and spring force which works best in anyparticular environment and speed range can be determined experimentally.

It should be understood that the foregoing description is for thepurpose of illustration and that the invention includes allmodifications within the scope of the appended claims.

I claim:

1. A multiphase rotary stepping motor comprising a shaft;

a plurality of motor phases distributed axially along said shaft, eachof said phases comprising a rotor portion rotationally and axiallysecured to said shaft and having radially extending pole pieces, and astator portion around said rotor portion and having a coil and polepieces, said stator pole pieces being alignable with and separated bygaps from their associated rotor pole pieces for developing a flux paththerethrough upon passage of current in said coil;

housing means secured to said stator portions; and

bearing means associated with said housing means for mounting saidshaft, said bearing means comprising an axially fixed bearing located atone end of said motor phases for axially positioning said stator androtor pole pieces with respect to each other and for restricting axialmotion of said shaft at said one end, said axially fixed bearing being aduplex bearing having separate single bearings in face to facerelationship;

an axially floating bearing located at the other end of said motorphases and permitting axial motion of said shaft and said rotor polepieces over a preselected distance, said gaps accommodating the axialmotion of said rotor pole pieces over said preselected distance.

2. A multiphase rotary stepping motor comprising a shaft;

a plurality of phases each comprising a rotor portion secured to saidshaft and having rotor pole pieces, and a stator portion around saidrotor portion and having stator pole pieces alignable with said rotorpole pieces; and

damping means mounted on said shaft for absorbing energy imparted tosaid shaft and causing substantial acceleration thereof, said dampingmeans including a flange rotating said shaft;

a floating annular mass rotatable around said shaft; and

means frictionally interengaging said flange and mass and permittingslippage therebetween upon acceleration of said shaft.

3. A multiphase rotary stepping motor according to claim 2 wherein saiddamping means comprises a sleeve carrying said flange and mounted onsaid shaft, said annular masscomprising a pair of members on oppositesides of said sleeve flange and being resiliently urged together, saidfrictionally interengaging means comprising a pair of friction reducingdisks disposed in contact with the sides of said sleeve flange.

4. A multiphase rotary stepping motor according to claim 3 wherein theannular mass members resiliently urged against said sleeve flangecomprise a flange on said annular mass, and a washer disposed withinsaid annular mass and being resiliently urged toward said flanges by aspring.

5. A multiphase rotary stepping motor comprising a shaft;

a plurality of phases each comprising a rotor portion secured to saidshaft and having rotor pole pieces,

extending radially inward and adjacent to said sleeve flange, and asecond portion secured to said first portion, an axial compressionspring disposed between said first and second portions, and a washerbetween said sleeve flange and said spring; and

disks of friction reducing material between said sleeve flange and saidmass flange and between said sleeve flange and said washer.

1. A multiphase rotary stepping motor comprising a shaft; a plurality ofmotor phases distributed axially along said shaft, each of said phasescomprising a rotor portion rotationally and axially secured to saidshaft and having radially extending pole pieces, and a stator portionaround said rotor portion and having a coil and pole pieces, said statorpole pieces being alignable with and separated by gaps from theirassociated rotor pole pieces for developing a flux path therethroughupon passage of current in said coil; housing means secured to saidstator portions; and bearing means associated with said housing meansfor mounting said shaft, said bearing means comprising an axially fixedbearing located at one end of said motor phases for axially positioningsaid stator and rotor pole pieces with respect to each other and forrestricting axial motion of said shaft at said one end, said axiallyfixed bearing being a duplex bearing having separate single bearings inface to face relationship; an axially floating bearing located at theother end of said motor phases and permitting axIal motion of said shaftand said rotor pole pieces over a preselected distance, said gapsaccommodating the axial motion of said rotor pole pieces over saidpreselected distance.
 2. A multiphase rotary stepping motor comprising ashaft; a plurality of phases each comprising a rotor portion secured tosaid shaft and having rotor pole pieces, and a stator portion aroundsaid rotor portion and having stator pole pieces alignable with saidrotor pole pieces; and damping means mounted on said shaft for absorbingenergy imparted to said shaft and causing substantial accelerationthereof, said damping means including a flange rotating said shaft; afloating annular mass rotatable around said shaft; and meansfrictionally interengaging said flange and mass and permitting slippagetherebetween upon acceleration of said shaft.
 3. A multiphase rotarystepping motor according to claim 2 wherein said damping means comprisesa sleeve carrying said flange and mounted on said shaft, said annularmass comprising a pair of members on opposite sides of said sleeveflange and being resiliently urged together, said frictionallyinterengaging means comprising a pair of friction reducing disksdisposed in contact with the sides of said sleeve flange.
 4. Amultiphase rotary stepping motor according to claim 3 wherein theannular mass members resiliently urged against said sleeve flangecomprise a flange on said annular mass, and a washer disposed withinsaid annular mass and being resiliently urged toward said flanges by aspring.
 5. A multiphase rotary stepping motor comprising a shaft; aplurality of phases each comprising a rotor portion secured to saidshaft and having rotor pole pieces, and a stator portion around saidrotor portion and having stator pole pieces alignable with said rotorpole pieces; and damping means mounted on said shaft for absorbingenergy imparted to said shaft and causing substantial accelerationthereof, said damping means comprising a sleeve secured to said shaft; aradially extending flange integral with said sleeve; an annular masshaving a first portion with a flange extending radially inward andadjacent to said sleeve flange, and a second portion secured to saidfirst portion, an axial compression spring disposed between said firstand second portions, and a washer between said sleeve flange and saidspring; and disks of friction reducing material between said sleeveflange and said mass flange and between said sleeve flange and saidwasher.